[20/45] drivers/char/mem.c: avoid OOM lockup during large reads from /dev/zero
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From: Greg KH on 30 Mar 2010 20:40 2.6.27-stable review patch. If anyone has any objections, please let us know. ------------------ From: Salman Qazi <sqazi(a)google.com> commit 730c586ad5228c339949b2eb4e72b80ae167abc4 upstream. While running 20 parallel instances of dd as follows: #!/bin/bash for i in `seq 1 20`; do dd if=/dev/zero of=/export/hda3/dd_$i bs=1073741824 count=1 & done wait on a 16G machine, we noticed that rather than just killing the processes, the entire kernel went down. Stracing dd reveals that it first does an mmap2, which makes 1GB worth of zero page mappings. Then it performs a read on those pages from /dev/zero, and finally it performs a write. The machine died during the reads. Looking at the code, it was noticed that /dev/zero's read operation had been changed by 557ed1fa2620dc119adb86b34c614e152a629a80 ("remove ZERO_PAGE") from giving zero page mappings to actually zeroing the page. The zeroing of the pages causes physical pages to be allocated to the process. But, when the process exhausts all the memory that it can, the kernel cannot kill it, as it is still in the kernel mode allocating more memory. Consequently, the kernel eventually crashes. To fix this, I propose that when a fatal signal is pending during /dev/zero read operation, we simply return and let the user process die. Signed-off-by: Salman Qazi <sqazi(a)google.com> Cc: Nick Piggin <nickpiggin(a)yahoo.com.au> Signed-off-by: Andrew Morton <akpm(a)linux-foundation.org> [ Modified error return and comment trivially. - Linus] Signed-off-by: Linus Torvalds <torvalds(a)linux-foundation.org> Signed-off-by: Greg Kroah-Hartman <gregkh(a)suse.de> --- drivers/char/mem.c | 3 +++ 1 file changed, 3 insertions(+) --- a/drivers/char/mem.c +++ b/drivers/char/mem.c @@ -724,6 +724,9 @@ static ssize_t read_zero(struct file * f written += chunk - unwritten; if (unwritten) break; + /* Consider changing this to just 'signal_pending()' with lots of testing */ + if (fatal_signal_pending(current)) + return written ? written : -EINTR; buf += chunk; count -= chunk; cond_resched(); -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ |